1. Field of the Invention
This invention relates to an illuminator for use primarily in conjunction with semiconductor fabrication equipment and, more specifically, to a hybrid illuminator for use in bonding operations in conjunction with semiconductor fabrication.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, one of the operations required involves the bonding of wires between two bonding locations, generally between bond pads on the semiconductor device and lead fingers of a lead frame. This bonding operation requires the use of a wire bonder. A wire bonder uses fine gold wire to couple, for example but not limited to, the bond pads on the semiconductor chip to the individual leads or pins to which connection is to be made, such as, but not limited to, the lead fingers of the lead frame. With the continued miniaturization of the geometry of semiconductor devices as well as the components within and on the semiconductor devices, it has become increasingly difficult to accurately locate the bond pads and leads or pins to which bonds must be made. Bond pads are now approaching dimensions of 5 mils or 25 microns. It is necessary that the bonder know precisely where the connections are to be made on a chip by chip basis because the bond locations can differ from chip to chip, even on essentially identical chips. Even minute dimensional differences can result in failures (reduced yield) or diminished electrical properties of the final device.
To mitigate this problem, a machine-vision system was developed to locate the exact positions where these connections or bonds are to be made. The vision system automatically magnifies, displays, measures and locates different parts of the device at which bonds are to be made. This vision system includes mainly electronics, software, camera, optics and illumination units. Since ambient light is generally insufficient to properly illuminate the chip and lead frame components to which the bond is to be made, illumination units are required to provide the required light to make the device to which a bond is to be made sufficiently visible while the camera takes pictures through the optics. These pictures are digitally stored in the electronics hardware and are digitally processed by the software in well known manner which forms no part of this invention.
To view different parts of the object, different types of lighting are required. In general, an axial light is used to view the die surface because the die surface is generally very smooth and reflects incident light without scattering. Accordingly, incident light directed parallel to an optical axis normal to the surface of the die is reflected back almost entirely parallel to the optical axis. An off axis light is used to view the lead frame which has a generally rough surface and scatters light impinging thereon. The light on the lead frame must be applied evenly to avoid dark spots in the picture, the dark spots (e.g. shadows) introducing features in the pictures which are not actually a part of the structure being viewed and are therefore undesirable. Accordingly, for viewing of the lead frame, incident light comes from around the object and is diffused so that it scatters in all directions and at all angles. The amount of scattering depends upon the surface roughness at the edge of the light conductor from which the light exits the light conductor.
The axial light travels through a beam splitter to make the incident light follow the optical axis. However, a side effect of using the beam splitter is the reduction of optical efficiency of the system. As can be seen with reference to FIG. 1, half of the incident light from the light source to the beam splitter passes through the beam splitter and half is reflected therefrom and is wasted. In addition, half of the reflected light from the object is reflected by the beam splitter and half of that reflected light passes through the beam splitter and is wasted. The result is that, at most, only twenty-five percent of the original light from the axial light source is reflected to the camera. With such a large amount of light being lost, it is necessary to generate extremely large amounts of incident light initially which, in turn, requires a relatively larger and more powerful light source than would be required were a larger percentage of the incident light to be used. The result is a light source which is expensive, short lived and difficult to control.